_Currents and Counter-currents_ was published in 1861, and _Border-lines of Knowledge_ in 1862. The two latter books deal with scientific subjects, but are written in such an attractive style that they have been extremely popular not only with students but with the whole reading public. _Songs in many Keys_, a volume of poems dedicated to his mother, was published by Doctor Holmes in 1862. _Mechanism in Thoughts and Morals_ appeared in 1871, the same year that _The Poet at the Breakfast-Table_ was running as a serial in the _Atlantic Monthly_, and numerous stray poems were also written in this prolific decade. In 1872 the poet's breakfast talk was published in book form. It is interesting to compare these three volumes--The Autocrat, the Professor, and the Poet. As a series they are as necessary to one another as the three strands of a cable, and yet each volume is, in a certain way, completed in itself. Where in the whole range of the English language, or indeed, of any language, will you find such an overflow of spontaneous wit and humor? While in no sense a story or even a narrative, the breakfast talk is enlivened by wonderfully life-like characters. We can easily imagine ourselves sitting beside them at the social table, and just as it is in real life, these chance acquaintances touch us at different points, awaken various degrees of interest, and are at all times quite distinct from the observer's own individuality. There is not a page without its sparkle of humor, and nugget of sound philosophy beneath, which the reader appropriates to himself in a delightfully unconscious manner--for the time being, it is he who is the Autocrat, the Professor, the Poet! As some one has truly said, "It is our thoughts which Doctor Holmes speaks; it is our humor to which he gives expression; it is the pictures of our own fancy that he clothes in words, and shows us what we ourselves thought, and only lacked the means of expressing. We never realized until he taught us by his magic power over us, how much each of us had of genius and invention and expression." Each book has its little romance, and the "Poet" introduces a poor gentlewoman whose story interests us quite as much as does that of the two lovers. "In a little chamber," he says, "into which a small thread of sunshine finds its way for half an hour or so every day during a month or six weeks of the spring or autumn, at all other times obliged to content itself with ungilded daylight, lives this boarder, whom, without wronging any others of our company, I may call, as she is very generally called in the household, the Lady....